Tetrapyrrolic cofactors participate in many critical cellular functions such as respiration (heme), photosynthesis (chlorophyll), sulfite and nitrite reduction (siroheme) and mammalian methionine synthesis (vitamin B12). In humans, a vitamin B12 deficiency contributes to the disease pernicious anemia, and the inappropriate build up of heme precursors causes various types of skin and blood disorders called porphyrias. The complex biosynthetic pathways of these cofactors start with common precursors, and branch at various stages of modification, with the longest and most complex pathway belonging to vitamin Bl2 (Scott, 1993; Battersby, 1994). One multifunctional enzyme participates at the crossroads of vitamin Bl2 and siroheme biosynthesis, siroheme synthase. This enzyme is responsible for three separate enzymatic reactions, porphyrin ring methylation, dyhdrogenation, and iron chelation. The three dimensional structure of siroheme synthase will be solved by X-ray diffraction techniques. Substrate and cofactor complexes will provide a detailed view of the critical interactions necessary for ligand binding. The catalytic mechanism will be probed by site directed mutagenesis to determine the function of residues identified in the structure to hold critical positions around the active sites.